It is known that red phosphorus upon storage in a moist atmosphere, undergoes a chemical reaction with the resultant-formation of various acids of phosphorus, mainly phosphorous acid and phosphoric acid, and phosphine. The formation of the highly toxic phosphine gives rise to hazardous working conditions and the formation of the phosphorus and phosphoric acids is undesirable in end uses of red phosphorus. Aluminum in the form of its hydroxide has been widely used to stabilize red phosphorus against such oxidation. However, relatively large amounts of aluminum are required to achieve a significant degree of stabilization.
An additional problem which arises with the prior art aluminum treatment is that the product is difficult to process. A layer of alumina is precipitated onto the red phosphorus particles in an aqueous dispersion thereof and then the treated red phosphorus is filtered and dried. Efficient filtration of the treated red phosphorus is difficult to achieve as a result of gelation of the aluminum hydroxide and large quantities of water are retained by the aluminum hydroxide.
Recently, it has been suggested in U.S. Pat. No. 4,115,522 to effect stabilization using orthophosphoric acid salts of aluminum, magnesium, calcium or zinc. The aluminum salt is the most effective and, again, relatively large quantities are required.
More recently, U.S. Pat. No. 4,210,630 suggests that improved stabilization can be achieved by using lead hydroxide in combination with aluminum hydroxide. A further suggestion, set forth in Canadian Pat. No. 1,097,152, is to superficially cover each red phosphorus particle with a thin film of a hardened melamine-formaldehyde resin.